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Weather is the state of the atmosphere, describing for example the degree to which it is hot or cold, wet or dry, calm or stormy, clear or cloudy. On Earth, most weather phenomena occur in the lowest layer of the planet's atmosphere, the troposphere, just below the stratosphere. Weather refers to day-to-day temperature, precipitation, and other atmospheric conditions, whereas climate is the term for the averaging of atmospheric conditions over longer periods of time. When used without qualification, "weather" is generally understood to mean the weather of Earth.

Weather is driven by air pressure, temperature, and moisture differences between one place and another. These differences can occur due to the Sun's angle at any particular spot, which varies with latitude. The strong temperature contrast between polar and tropical air gives rise to the largest scale atmospheric circulations: the Hadley cell, the Ferrel cell, the polar cell, and the jet stream. Weather systems in the middle latitudes, such as extratropical cyclones, are caused by instabilities of the jet streamflow. Because Earth's axis is tilted relative to its orbital plane (called the ecliptic), sunlight is incident at different angles at different times of the year. On Earth's surface, temperatures usually range ±40 °C (−40 °F to 104 °F) annually. Over thousands of years, changes in Earth's orbit can affect the amount and distribution of solar energy received by Earth, thus influencing long-term climate and global climate change.

Surface temperature differences in turn cause pressure differences. Higher altitudes are cooler than lower altitudes, as most atmospheric heating is due to contact with the Earth's surface while radiative losses to space are mostly constant. Weather forecasting is the application of science and technology to predict the state of the atmosphere for a future time and a given location. Earth's weather system is a chaotic system; as a result, small changes to one part of the system can grow to have large effects on the system as a whole. Human attempts to control the weather have occurred throughout history, and there is evidence that human activities such as agriculture and industry have modified weather patterns.

Studying how the weather works on other planets has been helpful in understanding how weather works on Earth. A famous landmark in the Solar System, Jupiter's Great Red Spot, is an anticyclonic storm known to have existed for at least 300 years. However, the weather is not limited to planetary bodies. A star's corona is constantly being lost to space, creating what is essentially a very thin atmosphere throughout the Solar System. The movement of mass ejected from the Sun is known as the solar wind. (Full article...)

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Extratropical cyclones, sometimes called mid-latitude cyclones, are a group of cyclones that occur in the middle latitudes of the Earth, and which have neither tropical nor polar characteristics. They are connected with fronts and feature changes in temperature and dew point horizontally, otherwise known as "baroclinic zones". Extratropical cyclones are the everyday phenomena which, along with anticyclones, drive much of the weather on Earth, producing anything from cloudiness and mild showers to heavy gales and thunderstorms. The image on right is a picture taken by a weather satellite in infrared of the 1993 North American Storm Complex, an extremely strong extratropical cyclone known to many in North America as the "Storm of the Century".

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A Von Kármán vortex street is a repeating pattern of swirling vortices caused by the unsteady separation of flow over blunt bodies. They are named after the engineer and fluid dynamicist, Theodore von Kármán. These vortices can appear on a large scale in nature when unidirectional boundary layer wind flows around isolated hills or islands, and can be visible from space if a layer of clouds are present. In this satellite picture of one of the Juan Fernández Islands, the wind at cloud level is moving from lower left to upper right, and the island is in the clear area at the lower left.

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More did you know...

...that the Flying river is the name given to the transport of water vapor from the Amazon rainforest to southern Brazil?

...that hurricane shutters are required for all homes in Florida unless impact-resistant glass is used?

...that the Joint Institute for Marine and Atmospheric Research is a combined weather and ocean research institute with the cooperation of the Office of Oceanic and Atmospheric Research and the University of Hawaiʻi?

...that the SS Central America was sunk by a hurricane while carrying more than 30,000 pounds (13,600 kg) of gold, contributing to the Panic of 1857?

...that a hurricane force wind warning is issued by the United States National Weather Service for storms that are not tropical cyclones but are expected to produce hurricane-force winds (65 knots (75 mph; 120 km/h) or higher)?

...that the Automated Tropical Cyclone Forecasting System is a software package for tropical cyclone forecasting developed in 1988 that is still used today by meteorologists in various branches of the US Government?

Recent and ongoing weather

This week in weather history...

July 21

1961: Hurricane Anna reached peak intensity over the Caribbean Sea, with peak winds of 115 miles per hour (185 km/h).

July 22

1342: St. Mary Magdalene's flood, the worst flooding in central European history, reached its peak, inundating most rivers and valleys across central Europe.

July 23

2010: The largest hailstone ever measured (by diameter) fell near Vivian, South Dakota. The hailstone was measured at 8.0 inches (20 cm) across.

July 24

1959: Hurricane Debra made landfall near Galveston, Texas, bringing more than 15 inches (380 mm) of rain in Orange County that resulted in severe flooding.

July 25

2019: A historic heat wave reached its peak across western Europe, with temperatures reaching all-time record highs in Germany (42.6 °C, 108.7 °F), Belgium (41.8 °C, 107.2 °F), Luxembourg (40.8 °C, 105.4 °F), and the United Kingdom (38.7 °C, 101.7 °F).

July 26

2005: Almost 1,000 millimeters (39 in) of rain in just 24 hours caused severe flooding in the Indian state of Maharashtra, killing more than 1000 people.

July 27

1943: Colonel Joe Duckworth became the first man to fly into the eye of a tropical cyclone, taking his AT-6 Texan training plane into the 1943 Surprise Hurricane twice.

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Tetsuya Theodore Fujita (/fˈtɑː/; FOO-jee-tah) (藤田 哲也, Fujita Tetsuya, October 23, 1920 – November 19, 1998) was a Japanese-American meteorologist whose research primarily focused on severe weather. His research at the University of Chicago on severe thunderstorms, tornadoes, hurricanes, and typhoons revolutionized the knowledge of each. Although he is best known for creating the Fujita scale of tornado intensity and damage, he also discovered downbursts and microbursts and was an instrumental figure in advancing modern understanding of many severe weather phenomena and how they affect people and communities, especially through his work exploring the relationship between wind speed and damage. (Full article...)

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The scope of WikiProject Weather is to have a single location for all weather-related articles on Wikipedia.

WikiProject Meteorology is a collaborative effort by dozens of Wikipedians to improve the quality of meteorology- and weather-related articles. If you would like to help, visit the project talk page, and see what needs doing.

WikiProject Severe weather is a similar project specific to articles about severe weather. Their talk page is located here.

WikiProject Tropical cyclones is a daughter project of WikiProject meteorology. The dozens of semi-active members and several full-time members focus on improving Wikipedia's coverage of tropical cyclones.

WikiProject Non-tropical storms is a collaborative project to improve articles related to winter storms, wind storms, and extratropical cyclones.

Wikipedia is a fully collaborative effort by volunteers. So if you see something you think you can improve, be bold and get to editing! We appreciate any help you can provide!

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